Vol. 60 No. 5 Sept-Oct (2014): Revista Mexicana de Física.

Published: 2014-01-01

Articles

  • Origin of conical dispersion relations

    Sergio A. Hojman
    336-0
    Abstract:
    A mechanism that produces conical dispersion relations is presented. A Kronig Penney one dimensional array with two different strengths delta function potentials gives rise to both the gap closure and the dispersion relation observed in graphene and other materials. The Schrödinger eigenvalue problem is locally invariant under the infinite dimensional Virasoro algebra near conical dispersion points in reciprocal space, thus suggesting a possible relation to string theory.
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  • Caracterización por SEM, EDS y micro-Raman de la influencia de la relación molar SiO$_2$/Al$_2$O$_3$ y temperatura de reducción sobre el sistema CuZSM5

    F. Chávez Rivas, I. Rodríguez Iznaga, V. Petranovskii
    340-0
    Abstract:
    The morphological and spectroscopic changes caused by reduction of ion-exchanged CuZSM5 samples with SiO$_2$/Al$_2$O$_3$ molar ratios (MR) of 30 and 70 in H$_2$ flow at 150 and 350$^{\circ}$C are discussed. The nominal copper loading was of 1% by weight. SEM microscopy of CuZSM5 sample set showed that the treatment in reducing atmosphere at 350$^{\circ}$C produces particle agglomeration and smoother surface texture that treatment at 150$^{\circ}$C; this effect was more pronounced for the sample with RM 30. Furthermore, the particles corresponding to the sample with RM 30 are of irregular shape and have rough texture (cubic or rectangular crystals, protrusions) while the particles of the sample with RM 70 are in the majority of spherical smooth texture (leaf-shaped crystals/platelet, reclining). EDS elemental analysis reveals a heterogeneous distribution of copper as well other elements in each sample. The micro-Raman spectroscopy shows some changes in the vibrational modes for the studied samples.
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  • Coulomb effects in isobaric cold fission from reactions $^{233}$U($n_{\rm{th}}$,f), $^{235}$U($n_{\rm{th}}$,f), ${}^{239}$Pu($n_{\rm{th}}$,f) and $^{252}$Cf(sf)

    M. Monto, a.
    350-0
    Abstract:
    The Coulomb effect hypothesis, formerly used to interpret fluctuations in the curve of maximal total kinetic energy as a function of light fragment mass in reactions $^{233}$U($n_{\rm{th}}$,f), $^{235}$U($n_{\rm{th}}$,f) and $^{239}$Pu($n_{\rm{th}}$,f), is confirmed in high kinetic energy as well as in low excitation energy windows, respectively. Data from reactions $^{233}$U($n_{\rm{th}}$,f), $^{235}$U($n_{\rm{th}}$,f), ${}^{239}$Pu($n_{\rm{th}}$,f) and $^{252}$Cf(sf) show that, between two isobaric fragmentations with similar $Q$-values, the more asymmetric charge split reaches the higher value of total kinetic energy. Moreover, in low excitation energy windows, between two isobaric charge splits with different $Q$-values, the more asymmetrical fragmentations is preferred.
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  • Quasinormal frequencies of the Dirac field in a $D$-dimensional Lifshitz black hole

    A. López-Ortega
    357-0
    Abstract:
    In a $D$-dimensional Lifshitz black hole we calculate exactly the quasinormal frequencies of a test Dirac field in the massless and zero angular eigenvalue limits. These results are an extension of the previous calculations in which the quasinormal frequencies of the Dirac field are determined, but in four dimensions. We discuss the four-dimensional limit of our expressions for the quasinormal frequencies and compare with the previous results. We also determine whether the Dirac field has unstable modes in the $D$-dimensional Lifshitz spacetime.
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  • Enhancement of vectorial nonlinearity in rubidium vapor by using an additional pump beam

    N. Korneev, Y.M. Torres, Y. Ortega
    366-0
    Abstract:
    We report the enhancement of nonlinearity to absorption ratio in rubidium vapor for a signal beam in ${}^{87}$Rb transition D2 line with $F_g=1$ by means of an additional pump beam. Experimental results for a pump beam in co- and counter-propagation configurations are reported. The ratio increment of approximately 2 times is obtained for the co-propagating case when the pump beam is tuned at $F_g=1$.
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  • Higher dimensional Elko theory

    J. A
    371-0
    Abstract:
    We show that the so-called Elko equation can be derived from a $5$% -dimensional Dirac equation. We argue that this result can be relevant for dark matter and cosmological scenarios. We generalize our procedure to higher dimensions.
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  • Probabilidad de transmisión en cadenas desordenadas de ADN

    C. A, K. M, R. R, -González.
    376-0
    Abstract:
    In the last years have grown the interest to use nucleic acids and proteins in nanodevices. However, some their electronic properties remain unclear and there is still much more to be done. We address the problem of charge transport across a finite segment of DNA, using a linear chain model into tight binding framework with nearest neighbors interaction and one orbital per site to represent the DNA. Hydrations effects are taken into account as random variations of the self-energy. Transmission probability is calculated from transfer matrix concept.
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  • Heat engines and the Curzon-Ahlborn efficiency

    A. Calvo Hernández, J.M.M. Roco, A. Medina, N. Sánchez-Salas
    384-0
    Abstract:
    The so-called Curzon-Ahlborn efficiency is becoming a paradigmatic result with regards to thermodynamic optimization of power cycles. Its wide applicability and sole dependence on the external heat bath temperatures (as the Carnot efficiency does) allows for an easy and fairly comparison with experimental efficiencies of striking validity. Different analytical derivations are presented in order to assess its validity and limitations for a broad variety of thermal cycles and steady state systems based on Finite-Time, Linear-Irreversible and Equilibrium Thermodynamic frameworks. Some conclusions and future perspectives are also outlined.
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  • Preparation, structural and NLO-optical characterization of LB-molecular films from asymmetric Bent-Core liquid crystals

    P. García-Vázquez, O.G. Morales-Saavedra, M.D.P. Carreón-Castro, G. Pelzl
    390-0
    Abstract:
    Molecular mono- and multilayered films of a polar asymmetric bent-core (``banana-shaped'') liquid crystalline (LC) compound with hydrocarbon end-chains were prepared by the Langmuir-Blodgett (LB) technique. Langmuir films were characterized by surface pressure isotherms and Brewster angle microscopy (BAM). Likewise, LB-films deposited onto glass substrates were characterized by UV-VIS spectroscopy, the optical second harmonic generation (SHG) technique and atomic force microscopy (AFM). Results show that the asymmetric structure of bent-core liquid crystals may promotes an unstable multi-layered ($n > 10$ LB-layers) LB-architecture which leads to a rapid collapse of Z-type arrangements, giving rise to a drastic decrease of the nonlinear optical (NLO) properties and film quality. Indeed, measurements evidence a tolerable and uniform molecular coverage on the glass substrates with anisotropic orientational distribution for a moderate number of layers only ($n \leq 10$ LB-layers); where, according to NLO-experimental data, the net molecular polarization is aligned outward the substrate layer. This observation leads us to implement a simplified model based on the monomeric rod-like approximation, in order to estimate significant NLO-tensorial components and an effective molecular hyperpolarizability $\beta_{\textrm{eff}}$ -coefficient along the polar axes of the 2D-polar LC-compound within the mechanical stable LB-monolayer arrangements.
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  • Changes of representation and general boundary conditions for Dirac operators in 1+1 dimensions

    S. De Vincenzo
    401-0
    Abstract:
    We introduce a family of four Dirac operators in 1+1 dimensions: $\hat{h}_{A}=-i\hbar c\,\hat{\mathcal{\mathit{\mathrm{\Gamma}}}}_{A}\,\partial/\partial x$ ($A=1,2,3,4$) for $x\in\Omega=[a,b]$. Here, $\{\hat{\mathit{\mathrm{\Gamma}}}_{A}\}$ is a complete set of $2\times2$ matrices: $\hat{\mathit{\mathrm{\Gamma}}}_{1}=\hat{1}$, $\hat{\mathit{\mathrm{\Gamma}}}_{2}=\hat{\alpha}$, $\hat{\mathit{\mathrm{\Gamma}}}_{3}=\hat{\beta}$, and $\hat{\mathit{\mathrm{\Gamma}}}_{4}=i\hat{\beta}\hat{\alpha}$, where $\hat{\alpha}$ and $\hat{\beta}$ are the usual Dirac matrices. We show that the hermiticity of each of the operators $\hat{h}_{A}$ implies that $C_{A}(x=b)=C_{A}(x=a)$, where the real-valued quantities $C_{A}=c\psi^{\dagger}\hat{\mathit{\mathrm{\Gamma}}}_{A}\psi$, the bilinear densities, are precisely the components of a Clifford number $\hat{C}$ in the basis of the matrices $\hat{\mathcal{\mathit{\mathrm{\Gamma}}}}_{A}$; moreover, $\hat{C}/2c\varrho$ is a density matrix ($\varrho$ is the probability density). Because we know the most general family of self-adjoint boundary conditions for $\hat{h}_{2}$ in the Weyl representation (and also for $\hat{h}_{1}$), we can obtain similar families for $\hat{h}_{3}$ and $\hat{h}_{4}$ in the Weyl representation using only the aforementioned family for $\hat{h}_{2}$ and changes of representation among the Dirac matrices. Using these results, we also determine families of general boundary conditions for all these operators in the standard representation. We also find and discuss connections between boundary conditions for the free (self-adjoint) Dirac Hamiltonian in the standard representation and boundary conditions for the free Dirac Hamiltonian in the Foldy-Wouthuysen representation.
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